Most of us don’t think about what holds stairs up when we walk up and down them. Inside the house, you might guess it was the walls below the stairs, but actually stairs are self-supporting structures that are held up by their stringers, the long pieces to which each tread is fastened. Each stringer is like the leg of a ladder. Each tread is like a ladder’s rung. This self-supporting nature becomes clear when you look at outdoor stairs that serve a deck, where the stringers are fully visible.

Expert Tip

Because deck stairs may land on uneven ground or on a wavy concrete pad, the stringers eventually move out of alignment. Check across the tread cut on each stringer using a level, and be prepared to shim the foot of the stairs to bring the stringers into alignment.